1. Field of the Invention
The present invention relates to an apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
An apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web is used in so-called coating systems for providing one or both sides of a traveling material web, consisting for example of paper, cardboard or a textile material, with one or several layers of the coating medium, for example color, starch, impregnating fluid or the like. The coating medium may also contain acid or have other aggressive properties.
An apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard, is known from German Document No. 44 32 179 C1, assigned to the assignee of the present invention. This apparatus includes an oblong unit which is in direct contact with the coating medium, namely an open-jet nozzle applicator with a feed slot for the coating forming between a front wall arranged on the feed side and a back wall arranged on the leaving side. The apparatus also includes feed conduits by way of which the liquid medium is fed from a central manifold to the feed slot. Further, the apparatus has an oblong support element for the unit, namely a support beam that is joined integrally to the unit and includes the central manifold. Customarily, instead of this integral design, the working unit and the support element are made as separate parts from same materials and secured to one another by means of welding. The working unit and the support element usually have a length of several meters. In the so-called direct application, the liquid or pasty coating medium, as is known, is applied directly onto the surface of the traveling material web, the latter being carried during application on a rotating countersurface, for example an endless belt or a backing roll. In the indirect application of the medium, in contrast, the liquid or pasty coating medium is first applied onto a substrate, for example the surface of a backing roll configured as an applicator roll, and is transferred from the applicator roll to the material web in a nip through which the material web passes.
The coating medium pumped through the apparatus is normally hot as a result of proceeding through various components of the apparatus, passing through circulating devices, or due to the effect of either heating units or drying systems following the working unit. A considerable temperature difference occurs between the hot coating medium, the partial areas of the working unit heated by the coating medium, and the colder support element. The different length changes of these components resulting from the temperature differences create sort of a "bimetal effect" which causes a distortion or flexure of the apparatus structure with respect to the surface being coated and thus, with no suitable countermeasures being taken, appreciable inaccuracies of the applied coating.
Therefore, thermal, mechanical, pneumatic or hydraulic flexure compensation systems (known also as "ADS" or an "Antideflection System") are employed in conventional apparatuses. The systems counteract the above effect and achieve an improved dimensional accuracy of the application across the entire machine width. A support arrangement equipped with a thermal flexure compensation system wherein a temperature-modulating liquid passes through the support element provided with several chambers is described, e.g., in U.S. Pat. No. 3,134,126. Introducing the liquid, temperature-modulated appropriately via a temperature control, achieves a minimum flexure of the doctor beam.
The flexure compensation systems described above, however, are rather expensive in design and, consequently, cost-intensive. The systems also require appreciable expense for measurement and control of regulating technology.
In the prior apparatuses of the type described above the working unit (or major parts of it) and the support element are joined to one another fixedly. Hence, a material pairing adapted to the technique of joining these two components of the apparatus usually required identical materials, which mostly very much limits the designer with respect to material selection. Therefore, material pairings that would have a more favorable effect on the negative flexure behavior explained above, or utilizations of special materials for apparatus areas that are particularly stressed by the coating medium, are normally beyond realization with the prior technologies. Furthermore, the support element and major parts of the working unit associated with it must be fabricated jointly, presupposing large and expensive machinery systems. Lastly, a specific working unit always also requires the manufacture of an exactly adapted specific support element, resulting in higher manufacturing costs.